(Parallel-faced hemihedral).
The only elements of symmetry are the triad axis and a centre of
symmetry. The general form {hkl} is a rhombohedron, and is a
hemihedral form, with parallel faces, of the scalenohedron. The form
{hkl}, where h - 2k + l = 0, is also a rhombohedron, being the
hemihedral form of the hexagonal bipyramid. The dihexagonal prism
{hk'l'} of the holosymmetric class becomes here a hexagonal prism. The
rhombohedra (hkk), hexagonal prisms {21'1'} and {101'}, and the basal
pinacoid {111} are geometrically the same in this class as in the
holosymmetric class.
Fig. 76 represents a crystal of dioptase with the fundamental
rhombohedron r {100} and the hexagonal prism of the second order m
{101'} combined with the rhombohedron s {031'}.
Examples of minerals which crystallize in this class are phenacite,
dioptase, willemite, dolomite, ilmenite and pyrophanite: amongst
artificial substances is ammonium periodate ((NH4)4I2O9.3H2O).